Add `exclude` parameter to exclude specific paths

[afragen]

Is your feature request related to a problem? Please describe.
Adds the ability to exclude specific paths from the POT.

Describe the solution you'd like
Adds the --exclude parameter for wp i18n make-pot ...

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
I also fixed a bug where the slug was identified as $SLUG and not $ITEM_SLUG